Seattle, chief city of the state of Washington, U.S., seat (1853) of King county, the largest metropolis of the Pacific Northwest, and one of the largest and most affluent urban centres in the United States. A major port of entry and an air and sea gateway to Asia and Alaska, Seattle lies alongside Puget Sound, a deep inland arm of the northern Pacific Ocean, and is at the centre of a conurbation that is defined roughly by Everett to the north, Bellevue to the east, and Tacoma to the south.